Rationalism is any view appealing to intellectual and deductive reason (as opposed to sensory experience or any religious teachings) as the source of knowledge or justification. Thus, it holds that some propositions are knowable by us by intuition alone, while others are knowable by being deduced through valid arguments from intuited propositions. Depending on the strength of the belief, this can result in a range of positions from the moderate view that reason has precedence over other ways of acquiring knowledge, to the radical position that reason is the only path to knowledge.

Rationalism relies on the idea that reality has a rational structure in that all aspects of it can be grasped through mathematical and logical principles, and not simply through sensory experience. Rather than being a "tabula rasa" to be imprinted with sense data, the mind is structured by, and responds to, mathematical methods of reasoning.

Rationalists adopt at least one of three main claims:

Intuition/Deduction: Some propositions are knowable by us by intuition alone, whicle others are knowable by being deduced from intuited propositions. Some rationalists take intuition to be infallible, claiming that whatever we intuit must be true; others allow for the possibility of false intuited propositions. Some claim that only mathematics can be knowable by intuition and deduction; some that ethical truths can also be intuited; some more radical rationalists maintain that a whole range of metaphysical claims (like the existence of God, free will and the duality of mind and body) are include within the range of intuition and deduction.

Innate Knowledge: We have knowledge of some truths as part of our innate rational nature. Experiences may trigger a process by which we bring this knowledge to consciousness, but the experiences do not provide us with the knowledge itself, which has in some way been with us all along. Some rationalists claim that we gained this innate knowledge in an earlier existence, some that God provided us with it at creation, and others that it is part of our nature through natural selection.

Innate Concepts: Some of the concepts (as opposed to actual knowledge) we employ are part of our innate rational nature. Some would argue, however, that innate concepts are entailed by innate knowledge, because a particular instance of knowledge can only be innate if the concepts that are contained in the proposition are also innate.

Some rationalists also claim, in addition to the claims above, that the knowledge we gain by intuition and deduction, as well as the ideas and instances of knowledge that are innate to us, are indispensible and could not have been gained through sense experience, and/or that reason is superior to experience as a source of knowledge.

Rationalism is contrasted with Empiricism, the view that the origin of all knowledge is sense experience and sensory perception. It is usually associated with the introduction of mathematical methods into philosophy during the Age of Reason and the Enlightenment by the major rationalist figures, Descartes, Leibniz and Spinoza. It is commonly referred to as Continental Rationalism because it was predominant in the continental schools of Europe, whereas British Empiricism dominated in Britain.

The distinction between Rationalism and Empiricism, however, is perhaps not as clear-cut as is sometimes suggested, and would probably not have even been recognized by the Enlightenment philosophers involved. For example, the three main rationalists were all committed to the importance of empirical science, and in many respects the empiricists were closer to Descartes in their methods and metaphysical theories than were Leibniz and Spinoza. Both Leibniz and Spinoza asserted that, in principle, all knowledge, including scientific knowledge, could be gained through the use of reason alone, though they both observed that this was not possible in practice for human beings, except in specific areas such as mathematics.

René Descartes is one of the earliest and best known proponents of Rationalism. He believed that knowledge of eternal truths (e.g. mathematics and the epistemological and metaphysical foundations of the sciences) could be attained by reason alone, without the need for any sensory experience. Other knowledge (e.g. the knowledge of physics), required experience of the world, aided by the scientific method - a moderate rationalist position. For instance, his famous dictum "Cogito ergo sum" ("I think, therefore I am") is a conclusion reached a priori and not through an inference from experience. Descartes held that some ideas (innate ideas) come from God; others ideas are derived from sensory experience; and still others are fictitious (or created by the imagination). Of these, the only ideas which are certainly valid, according to Descartes, are those which are innate.

Baruch Spinozaexpanded upon Descartes' basic principles of Rationalism. His philosophy centred on several principles, most of which relied on his notion that God is the only absolute substance (similar to Descartes' conception of God), and that substance is composed of two attributes, thought and extension. He believed that all aspects of the natural world (including Man) were modes of the eternal substance of God, and can therefore only be known through pure thought or reason.

Gottfried Leibniz attempted to rectify what he saw as some of the problems that were not settled by Descartes by combining Descartes' work with Aristotle's notion of form and his own conception of the universe as composed of monads. He believed that ideas exist in the intellect innately, but only in a virtual sense, and it is only when the mind reflects on itself that those ideas are actualized.

Immanuel Kant started as a traditional rationalist, having studied Leibniz and Christian Wolff (1679 - 1754) but, after also studying the empiricistDavid Hume's works, he developed a distinctive and very influential Rationalism of his own, which attempted to synthesize the traditional rationalist and empiricist traditions.